A person's A B O type depends upon the presence of absence of two genes --the A and B genes. These genes are encoded on chromosome 9. They determine part of the configuration of the red blood cell surface.
A person can be A, B, AB, or O. If a person has two A genes, their red blood cells are type A. If a person has two B genes, their red cells are type B. If the person has one A and one B gene, their red cells are type AB. If the person has neither the A nor B gene, they are type O.
The situation with antibodies in blood plasma is just the opposite. Someone with type A red cells has anti-B antibodies (antibodies directed against type B red cells) in their blood plasma. Someone with type B red cells has anti-A antibodies in plasma. Someone who is type O has both anti-A and anti-B antibodies in plasma. And someone who is type AB has neither anti-A nor anti-B antibodies in plasma.
It is most important to determine the A B O status of both donor and recipient in transplants and transfusions by typing and cross-matching. A B O incompatibility in such procedures can be a disaster.
The first recorded blood transfusion may have taken in 1492.
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